There aren't any holistic graphics improvements, but the game can actually look pretty good on its own. Many custom modules look significantly better than the OC, especially when it comes to itemization. For UI, you can get extended hotbars and some complete redos on the vault, but nothing that would be considered an improved version of the original.
I also second rpgplayer1's spell fixes. I think the most recent spell fix pack is the one here: http://nwvault.ign.c...ts.Detail&id=99 which was updated a couple years ago. Careful with these, though, some of the changes (like sneak attack damage to negative level touch attacks) are ridiculously overpowered in the OC.
I'd highly recommend playing the first two campaigns. They're fun, voice acted, and open to whatever modding you want to install, particularly PRC packs and the like. Never liked SOZ. After that, there are some other great modules on the vault (dark waters comes to mind, but there are literally hundreds of hours worth of other ones). If multiplayer tickles your fancy, nwnlist.com is the way to find active servers. There are still active servers in the role playing, social, and story categories for both nwn1 and nwn2.